Generally, a planar solid electrolyte fuel cell (also referred to as a solid oxide fuel cell (SOFC)) is composed of a plurality of planar cells, as a power generating element, each composed of an anode (a negative electrode, a fuel electrode), a solid electrolyte and a cathode (a positive electrode, an air electrode) and interconnectors (also referred to as separators). The interconnectors are arranged between the plurality of cells in order to electrically connect the plurality of cells to each other in series and separate gases supplied to each of the plurality of cells; specifically, separate a fuel gas (e.g., hydrogen) as an anode gas supplied to an anode from an oxidant gas (e.g., air) as a cathode gas supplied to a cathode.
The interconnector needs to be chemically stable in a high-temperature environment of 800 to 1000° C. which is an operating temperature of the solid electrolyte fuel cell and in both oxidation and reduction atmospheres. Further, the interconnector material is desirably a material which has a high electric conductivity and can reduce an ohmic loss (IR loss).
In response to such requirements, the interconnector is conventionally formed of heat resistant metal materials or conductive ceramic materials such as lanthanum chromite (LaCrO3). When the interconnector is formed by use of such a conductive material, it is possible to make a member fulfilling the above-mentioned functions of electrical connection and separation of gases from one material. Generally, there have been used dense bodies of ceramic such as lanthanum chromite having a perovskite structure doped with Sr, Ca or Mg as interconnector materials.
However, when sintering lanthanum chromite in the air in the methods conventionally adopted in order to form an interconnector by using lanthanum chromite, chromium oxide is evaporated from the lanthanum chromite powder or compounds containing volatile hexavalent chromium are sintered in the process of vaporization/recondensation. Therefore, densification resulting from diffusion within a particle is inhibited and a gas-tight sintered body cannot be obtained.
In order to solve such problems, for example, a compound containing, as a dominant component, a composition represented by the composition formula La1-xCaxCr1-yO3 (values of x and y satisfy 0<x≦0.4, 0<y≦0.05, and y≦x) is proposed as a raw material powder of lanthanum chromite for forming a separator in Japanese Unexamined Patent Publication No. 4-119924 (hereinafter, referred to as Patent Document 1). Patent Document 1 indicates that in this raw material powder of lanthanum chromite, the amount of chromium evaporation can be reduced by making the chromium content insufficient and thereby the sintering property can be improved, and it becomes possible to realize a separator having an excellent gas-tight property, and to achieve chemical stability in the oxidation/reduction atmospheres and high electron conductivity, which are required for a separator.
Further, a compound containing, as a dominant component, a composition represented by the composition formula Sr1-xLaxTiO3 (the value of x satisfies 0<x≦0.3) is proposed, for example, in Japanese Unexamined Patent Publication No. 2001-52725 (hereinafter, referred to as Patent Document 2) as an interconnector material not containing Cr.
Moreover, a composition represented by the composition formula (La1-xSrx)(Fe1-yTiy)O3 (values of x and y satisfy 0≦x≦0.1 and 0<y<0.5) is proposed, for example, in Japanese Unexamined Patent Publication No. 2006-185697 (hereinafter, referred to as Patent Document 3) as an interconnector material which has a high sintering property, can be sintered at 1400° C. or lower, and does not contain Cr.